Install backported debootstrap

This debootstrap in the openstack-ci-core ppa is fixed so that Debian
buster can be built.

See

 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901977
 http://eavesdrop.openstack.org/irclogs/%23openstack-infra/%23openstack-infra.2019-04-03.log.html#t2019-04-03T21:25:50

Change-Id: Icb26b916ea21083777b05549a16c9b169c98b5aa

manifests/init.pp

@@ -21,20 +21,24 @@ class diskimage_builder (
   ) {
   include ::pip
 
+  # Install openstack-ci-core ppa
+  file { '/root/openstack-ci-core-ppa-key.pubkey':
+    ensure => present,
+    owner  => 'root',
+    group  => 'root',
+    mode   => '0644',
+    source => 'puppet:///modules/diskimage_builder/openstack-ci-core-ppa-key.pubkey',
+  }
+
+  # Key for openstack ppa repo; used below
+  include ::apt
+  apt::key { 'openstack-ci-core-ppa-key':
+    id      => '2B5DE24F0EC9F98BD2F85CA315B6CE7C018D05F5',
+    source  => '/root/openstack-ci-core-ppa-key.pubkey',
+    require => File['/root/openstack-ci-core-ppa-key.pubkey'],
+  }
+
   if $support_vhd {
-    file { '/root/openstack-ci-core-ppa-key.pubkey':
-      ensure => present,
-      owner  => 'root',
-      group  => 'root',
-      mode   => '0644',
-      source => 'puppet:///modules/diskimage_builder/openstack-ci-core-ppa-key.pubkey',
-    }
-    include ::apt
-    apt::key { 'openstack-ci-core-ppa-key':
-      id      => '2B5DE24F0EC9F98BD2F85CA315B6CE7C018D05F5',
-      source  => '/root/openstack-ci-core-ppa-key.pubkey',
-      require => File['/root/openstack-ci-core-ppa-key.pubkey'],
-    }
     apt::ppa { 'ppa:openstack-ci-core/vhd-util':
       require => Apt::Key['openstack-ci-core-ppa-key'],
     }
@@ -47,9 +51,20 @@ class diskimage_builder (
     }
   }
 
+  # Backported debootstrap that allows building Buster images on Xenial
+  apt::ppa { 'ppa:openstack-ci-core/debootstrap':
+    require => Apt::Key['openstack-ci-core-ppa-key'],
+  }
+  package { 'debootstrap':
+    ensure  => present,
+    require => [
+      Apt::Ppa['ppa:openstack-ci-core/debootstrap'],
+      Class['apt::update'],
+    ],
+  }
+
   $packages = [
     'debian-keyring',
-    'debootstrap',
     'kpartx',
     'python-lzma',
     'qemu-utils',